There are many known types of welding-type power supplies. Welding-type power, as used herein, refers to power suitable for electric arc welding, plasma cutting or induction heating. Welding-type systems are often used in a variety of applications and moved from one location to another. Consequently, portability is desirable, and the ability to accept different input voltages and frequencies is desirable. Welding-type system, as used herein, is a system that can provide welding type power, and can include control and power circuitry, wire feeders, and ancillary equipment.
Providing welding-type power, and designing systems to provide welding type power, provides for some unique challenges. For example, power supplies for most fields are dedicated to a single input, or are rarely moved from one input to another. But, welding type systems will often be moved from one location to another, and be used with different inputs, such as single or three phase, or 115V, 230V, 460V, 575V, etc., or 50 hz or 60 hz signals. Power supplies that are designed for a single input cannot provide a consistent output across different input voltages, and components in these power supplies that operate safely at a particular input level can be damaged when operating at an alternative input level. Also, power supplies for most fields are designed for relatively steady loads. Welding, on the other hand, is a very dynamic process and numerous variables affect output current and load, such as arc length, electrode type, shield type, air currents, dirt on the work piece, puddle size, weld orientation, operator technique, and lastly the type of welding process determined to be most suitable for the application. These variables constantly change, and lead to a constantly changing and unpredictable output current and voltage. Finally, power supplies for many fields are designed for low-power outputs. Welding-type power supplies are high power and present many problems, such as switching losses, line losses, heat damage, inductive losses, and the creation of electromagnetic interference. Accordingly, welding-type power supply designers face many unique challenges.
One prior art welding power supply that is well suited for portability and for receiving different input voltages is a multi-stage system with a preregulator to condition the input power and provide a stable bus, and an output circuit that converts or transforms the stable bus to a welding-type output. Examples of such welding-type systems are described in U.S. Pat. No. 7,049,546 (Thommes) and U.S. Pat. No. 6,987,242 (Geissler), and US Patent Publication 20090230941 (Vogel), all three of which are owned by the owner of this invention, and hereby incorporate by reference. Miller® welders with the Autoline® feature include some of the features of this prior art.
FIG. 1 shows a prior art three-phase welding-type power supply consistent with U.S. Pat. Nos. 7,049,546 and 6,987,242 and US Patent Publication 20090230941, and receives the three phase input Va, Vb and Vc on an input rectifier consisting of diodes 101-106. The rectified input is provided to a boost circuit 110, which boosts the input to a desired voltage (800V, e.g.) on a boosted or intermediate bus. Boost circuit 110 can include power factor correction, if desired. The boosted or intermediate bus is provided to a dc bus filter 112 (the bulk capacitance on the dc bus), and then to an isolated dc-dc converter 114. The dc-dc converter can include a converter (inverter, flyback, buck, etc), transformer and rectifier. The dc output is welding-type power. Such systems are significantly better than the prior art before them, and were the first welding-type systems to be “universal” in that they could accept nearly all available input power. They were also relatively portable and had improved power factors.
However, the three phase input rectifier commutates the line current between phases and forces discontinuous conduction. This occurs periodically—each time a line to line voltage exceeds the other two. As shown in FIGS. 2A-C, the current in each input line (Ia, Ib, & Ic) only conducts during two-thirds of the corresponding line cycle. The line current is zero for the remaining third, while the corresponding line voltage continues to follow the sinusoidal cycle above zero. As a result, the power factor, while better than other systems, is reduced because the line current no longer tracks the line voltage. FIGS. 2D-I show the current wave shape and conduction sequence of the three phase input rectifier diodes (101-106).
Operation of the three phase input rectifier may result in harmonic distortion of the line current. The harmonics distortion of the line current is driven by the abrupt non-sinusoidal step changes between zero and non-zero values as it commutates between phases.
Such prior art systems can also be designed for single or three phase operation, but are sometimes de-rated for single phase operation to achieve design goals for size, weight, and cost. The power circuit components must otherwise be designed for the additional current stresses required to meet rating during single phase operation. At the same power output, the line current required for single phase operation is increased by a √3 factor compared with the line current required for three phase operation.
The total power processed by such prior art systems is processed by a single power converter. Thus the power switch or input disconnecting device must be designed for the total power supply input current. Also parasitic inductances are increased by commonly used power semiconductor modules and by packaging constraints of physically larger components. These inductances are excited with higher switching currents, resulting in lower practical switching frequencies. Increased power dissipation is typically concentrated within larger individual components. This compromises the efficiency of the thermal design by localizing heat sources to relatively small spaces within the total volume of the power supply.
Accordingly, a welding-type system that maintains the advantages of prior art portable, universal input systems, but also avoids some of the deficiencies of the prior art is desired.